1. Field of the Invention
The present invention relates to a method and a device for adjusting magnetic field bias in a modulation drive circuit for a superconducting quantum interference device (hereinafter referred to as SQUID). In particular, the invention relates to a method and a device for easily adjusting magnetic field bias in a SQUID modulation drive circuit.
2. Description of the Background Art
FIG. 15 is a circuit diagram showing a conventional control circuit for a SQUID with a flux locked loop (hereinafter referred to as FLL). Referring to FIG. 15, the conventional SQUID FLL circuit includes a SQUID 81 having two Josephson junctions formed at predetermined positions and a constant current source 88 supplying constant current to SQUID 81. Magnetic flux to be measured is input from a pickup coil (not shown) to SQUID 81. Voltage output through both ends of SQUID 81 is converted by a transformer, amplified by a preamplifier 83 and then output from an output portion through a multiplier 84 and an integrator 85. A modulation signal at 40 kHz is added by a modulating unit 87 to the output from multiplier 84 for feedback to a field application coil 82 adjacent to SQUID 81. Accordingly, external magnetic flux detected at SQUID 81 is cancelled.
The output from SQUID 81 is also fed to an oscilloscope 90 provided separately from the output portion for observing the waveform thereof. Specifically, a feedback signal is partially input to the X axis of oscilloscope 90 and an output from preamplifier 83 is input to the Y axis of oscilloscope 90.
As a method of reducing low frequency noise, an AC biasing method is known. According to the AC biasing method, positive and negative currents having the same absolute value (the absolute value is equal to that of the constant current) are applied to a SQUID. In order to achieve a stable feedback, both outputs from the SQUID to which the positive and negative current biases are being applied should be in phase with each other. In order to achieve this, magnetic field bias is adjusted to match the phases with each other.
The magnetic field bias has conventionally been adjusted by manually adjusting the phase of voltage waveform on oscilloscope 90 while observing change of voltage relative to the magnetic field, with respect to time. This method has a problem that adjustment of the magnetic field bias is extremely complicated and time-consuming.
One object of the present invention is accordingly to provide a method of easily adjusting magnetic field bias in a modulation drive circuit for a SQUID.
Another object of the present invention is to provide a device for easily adjusting magnetic field bias in a modulation drive circuit for a SQUID.
The objects above are accomplished by a method of adjusting magnetic field bias in a SQUID modulation drive circuit that includes following steps. Specifically, according to one aspect of the present invention, a method of adjusting magnetic field bias in a SQUID modulation drive circuit includes the steps of applying a modulation signal at a predetermined first frequency to a flux locked loop maintaining magnetic flux supplied to the SQUID at a constant value, picking out an output voltage from the SQUID to which a predetermined bias current is being applied, and picking out the output voltage such that a frequency component of the voltage which is twice as high as the first frequency is a maximum frequency.
The output voltage is picked out from the SQUID to which a predetermined modulation frequency is being applied and then the frequency component of the output voltage twice the frequency of the applied predetermined modulation frequency is maximized, so that the magnetic field bias can be optimized easily.
Preferably, the step of applying to the flux locked loop the modulation signal at the predetermined first frequency includes the step of applying the modulation signal at the predetermined first frequency to a coil adjacent to the SQUID, and the magnetic field bias is thus adjusted by applying magnetic field bias to the coil adjacent to the SQUID.
A method of adjusting magnetic field bias can thus be provided with a simple structure since the magnetic field bias is adjusted by applying magnetic field bias to a coil adjacent to the SQUID, to which a modulation signal at a predetermined frequency is applied.
The magnetic field bias may be adjusted manually. A low cost method of adjusting magnetic field bias can accordingly be provided.
The magnetic field bias may be adjusted automatically. A labor-saving method of adjusting magnetic field bias can accordingly be provided.
The bias current may be alternating current or direct current.
According to another aspect of the invention, a device for adjusting magnetic field bias in a SQUID modulation drive circuit includes a unit for applying a modulation signal at a predetermined first frequency to a flux locked loop maintaining magnetic flux supplied to the SQUID at a constant value, a unit for picking out an output voltage from the SQUID to which a predetermined bias current is being applied, and a filter for picking out the output voltage such that a frequency component of the voltage which is twice as high as the first frequency is a maximum frequency.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.